The present invention pertains to steam turbines for utility power application and, more particularly, to a stationary blade for use in a low pressure steam turbine.
Steam turbine rotor and stationary blades are arranged in a plurality of rows or stages. The rotor blades of a given row are identical to each other and mounted in a mounting groove provided in the turbine rotor. Stationary blades, on the other hand, are mounted on a cylinder or blade ring which surrounds the rotor.
Turbine rotor blades typically share the same basic components. Each has a root receivable in the mounting groove of the rotor, a platform which overlies the outer surface of the rotor at the upper terminus of the root, and an airfoil which extends outwardly from the platform.
Stationary blades also have airfoils, except that the airfoils of the stationary blades extend downwardly towards the rotor. The airfoils include a leading edge, a trailing edge, a concave surface, and a convex surface. In most turbines, the airfoil shape common to a particular row of blades generally differs from the airfoil shape in other rows within a particular turbine. In general, no two turbines of different designs share airfoils of the same shape. The structural differences in airfoil shape result in significant variations in aerodynamic characteristics, stress patterns, operating temperature, and natural frequency of the blade. These variations, in turn, determine the expected life of the turbine blade under the operating conditions (turbine inlet temperature, pressure ratio, and rotational speed), which are generally determined prior to airfoil shape development.
Development of a turbine for a new commercial power generation steam turbine may require several years to complete. When designing rotor blades for a new steam turbine, a profile developer is given a certain flow field with which to work. The flow field determines the inlet angles (for steam passing between adjacent blades of a row), gauging, and the force applied on each blade, among other things. "Gauging" is the ratio of throat to pitch; "throat" is the straight line distance between the trailing edge of one blade and the suction surface of an adjacent blade, and "pitch" is the distance in the tangential direction between the trailing edges of the adjacent blades, each measurement being determined at a specific radial distance from the turbine axis.
These flow field parameters are dependent on a number of factors, including the length of the blades of a particular row. The length of the blades is established early in the design states of the steam turbine and is essentially a function of the overall power output of the steam turbine and the power output for that particular stage.
Referring to FIG. 1, two adjacent blades of a row are illustrated in sectional views to demonstrate some of the features of a typical blade. The two blades are referred to by the numerals 10 and 12. The blades have convex, suction-side surfaces 14 and 16, concave pressure-side surfaces 18 and 20, leading edges 22 and 24, and trailing edges 26 and 28.
The throat is indicated in FIG. 1 by the letter "O", which is the shortest straight line distance between the trailing edge of blade 10 and the suction-side surface of blade 12. The pitch is indicated by the letter "S", which represents the straight line distance between the trailing edges of the two adjacent blades.
The width of the blade is indicated by the distance W.sub.m, while the blade inlet flow angle is .alpha.1, and the outlet flow angle is .alpha.2.
".beta." is the leading edge included metal angle, and the letter "s" refers to the stagger angle.
When working with the flow field of a particular turbine, it is important to consider the interaction of adjacent rows of blades. The preceding row affects the following row by potentially creating a mass flow rate near the base which cannot pass through the following row. Thus, it is important to design a blade with proper flow distribution up and down the blade length.
The pressure distribution along the concave and convex surfaces of the blade can result in secondary flow which results in blading inefficiency. These secondary flow losses result from differences in steam pressure between the suction and the pressure surfaces of the blades near the end walls.
Regardless of the shape of the airfoil as dictated by the flow field parameters, the blade designer must also consider the cost of manufacturing the optimum blade shape. Flow field parameters may dictate a profile which cannot be produced economically, and inversely the optimum blade shape may otherwise be economically impractical. Thus, the optimum blade shape should also take into account manufacturability.